1. Field of the Invention:
The present invention relates to an automatic focusing camera equipped with an electromagnetically driven taking lens focusing device.
2. Description of the Prior Art:
Driving mechanisms of the motor-drivn system for focusing the lens unit of a camera and driving mechanisms of the electromagnetic driving system for the same purpose have been proposed. The former driving mechanisms are classified into driving mechanisms, as those employed in a lens shutter camera, which drive the taking lens to a position corresponding to a nearest range by a motor in conjunction with film winding operation after the completion of taking operation and move the taking lens to a position corresponding to a range signal by a spring immediately before the next taking operation, and driving mechanisms, as those employed as an automatic focusing device in a single-lens reflex camera, in which the direction and the angle of rotation of a motor for driving the taking lens are decided by a range signal. both those driving mechanisms need a reduction gear and mechanism for converting a rotary motion into a linear motion, and hence it has become difficult to build those conventional driving mechanisms in a compact and lightweight cameras in a recent years with the recent trend of cameras toward progressive reduction in size and weight.
Accordingly, the latter driving mechanisms, namely, electromagnetic driving mechanism, have often been employed in recent years. The electromagnetic driving mechanism for focusing the taking lens is provided with two permanent magnets fixedly disposed opposite to each other with a small gap therebetween to reduce loss in the magnetic circuit and a movable coil member disposed within the gap, and is adapted to drive the taking lens to bring the same into focus by the rotary or linear motion of the movable coil member. Therefore, the electromagnetic driving mechanism is subject to a restriction that the taking lens can be driven for focusing operation only in opposite directions according to the direction of an electric current, namely, from a position corresponding to a nearest range toward a position corresponding to an infinite range or from a position corresponding to an infinite range toward a position corresponding to a nearest range.
In an improved electromagnetic driving mechanism of the above-mentioned system, two movable coil members are disposed within the gap, one for adjusting the position of a cam surface for deciding the position of the taking lens and the other for moving the taking lens so that the taking lens is brought into abutment with the cam surface where the taking lens is focused. This electromagnetic driving mechanism is able to control the taking lens, at the most, in six steps between a position corresponding to a nearest range and a position corresponding to an infinite range. Furthermore, this electromagnetic driving mechanism has disadvantages that the construction is complicated and large.
Methods have been proposed as means to overcome those structural and functional disadvantages of the conventional electromagnetic driving mechanism, in which a signal provided by the range finder is converted into A pulse current, the movable coil member is caused to rock by the pulse current and the rocking motion of the movable coil member is transmitted to an external cam member to move the cam member stepwise.
According to one of such methods disclosed in Japanese Patent Laid-open No. 58-43434, the taking lens is controlled to a focus position in increased steps by an appropriate combination of the number of pulse signals given to the movable coil member and the number of steps of the external cam member.
However, since the cam member must be returned to the initial position after the completion of every taking operation, the construction of the mchanism is somewhat complicated and, furthermore, since a mechanism for moving the cam member stepwise is included in the electromagnetic driving mechanism, the cam surface needs to be formed within a limited space, and hence the cam surface is unable to have a sufficient length, which makes accurate focusing difficult.
Furthermore, in addition to the employment of an electromagnetic driving mechanism for focusing the taking lens, a trend of employing an electromagnetic shutter mechanism for controlling the operation of the shutter has been increased owing to the simple and compact construction of the electromagnetic shutter mechanism.
An electromagnetic driving mechanism for driving and controlling both a taking lens focusing mechanism and a shutter mechanism needs at least two movable coil members and those movable coil members must be disposed within a magnetic field of an appropriate flux density so that the movable coils are subjected to a sufficient electromagnetic force.
Accordingly, in the conventional electromagnetic driving mechanism, each movable coil member is disposed individually in a magnetic field formed exclusively for the same to enable the movable coil member to produce a sufficient driving force. This configuration, however, requires component parts of an increased number and a large space, which is disadvantageous to reducing the size and cost of a camera.
Japanese Utility Model Laid-Open No. 58-180538 discloses a mechanism for overcoming the above-mentioned disadvantages of the conventional electromagnetic driving mechanism. This mechanism is designed so as to reduce the number of parts and space by holding a plurality of permanent magnets in a row between a pair of yokes and movable coil members are disposed in magnetic fields formed on both sides of the permanent magnets respectively. This mechanism requires an extremely difficult work to align a plurality of permanent magnets accurately and the taking lens focusing operation and the shutter controlling operation cannot smoothly be carried out due to excessive unbalance in magnetic field intensity between the magnetic field on one side of the permanent magnets and that on the other side of the same, if the mechanism is not assembled accurately.
In a known electromagnetic driving mechanism, an electric current is supplied to a movable coil member disposed within a magnetic field formed by a permanent magnet and a yoke to rotate the movable coil member about the optical axis of the taking lens and the taking lens is focused or the shutter is controlled for exposure by the rotative force of the movable coil member.
When this known electromagnetic driving mechanism is employed in a taking lens focusing device, the movement of the movable coil member is converted into a movement along the optical axis of the taking lens, for shifting the taking lens to a position corresponding to a measured object distance. The taking lens unit disposed outside the electromagnetic driving mechanism is interlocked directly with the movable coil member or interlocked with the movable coil member by means of an elastic member, such as a spring, pressed against the taking lens unit. Since the movable coil member of such a taking lens focusing device must be capable of operating against the weight of the taking lens and the pressure of the elastic member, the movable coil member must have a large driving capacity, and hence a magnetic field of a high intensity needs to be produced. Consequently, such an electromagnetic driving mechanism is unavoidably large and expensive.
Japanese Patent Laid-Open No. 58-43434 discloses a mechanism to reduce the load on the electromagnetic driving mechanism. In this mechanism, the movable coil member functions only to regulate the focus controlling member, namely, only to set a member to limit the focusing movement of the taking lens, while the taking lens is driven for focusing by a separate driving source other than the electromagnetic driving mechanism. This mechanism reduces the load on the movable coil member remarkably and provides a compact and inexpensive electromagnetic driving mechanism facilitating the incorporation of the electromagnetic driving mechanism into a compact camera.
However, this mechanism employs an elastic member as a driving source for driving the taking lens to a focus position and the strained elastic member is released suddenly upon the execution of shutter releasing operation, therefore, the camera is liable to be shaked when the shutter is released. Furthermore, this mechanism needs a complex charging mechanism for returning the elastic member to the initial position after the completion of taking operation, which affects adversely to the accessibility and the construction of the camera, and hence the practical application of this mechanism is extremely difficult.